When asked how long the Apple Watch battery would last, Cook said it will vary based on how people use it. Still, he said users will have to charge the device nightly.

That's hardly reassuring. Cook's comment that battery life will depend on how you use it also leaves some wiggle room for heavy users. The watch might not even last a full day for them.

If the Apple Watch can't make it through a full day on a charge, it won't be very useful. It's one thing to charge your phone at your desk halfway through the day, but it's another thing to have to charge something strapped to your body. It's an inconvenient and clunky experience, one that we had when testing other smartwatches by manufacturers like Samsung and LG.

If the Apple Watch is going to serve its purpose, it'll have to last a little longer than 24 hours to provide the buffer needed to make it through a day of heavy use.

It's likely that Apple won't give an official stat on the watch's battery life until it launches early next year.